Using Alt + RMB drag to connect two nodes used to create an according mix node, linked both nodes to it and created a new link to the next node, if a link to that node already existed. Now it only connects the lower to the upper one, often even creating invalid links.
The class holding the operator is:
class NWLazyConnect(Operator, NWBase):
bl_idname = "node.nw_lazy_connect"
I noticed this still worked when called from the menu with 2 nodes selected. So as a hack/workaround on release I now call the operator used by the menu without parameters. That seems to work fine.
I have not managed to change the color of the outline and the gizmo, so it's always red, no matter if you are using 2 valid nodes, or not.